Frequency modulated radar level gauging

ABSTRACT

A radar level gauge and a method for measurement of a distance to a surface of a product in a tank are disclosed. The radar level gauge comprises transceiver circuitry configured to transmit and receive electromagnetic signals, the transceiver circuitry comprises a frequency stabilizing feedback loop configured to generate the electromagnetic transmit signal in the form of a frequency sweep. The frequency stabilizing feedback loop is configured to generate an oscillation, forming a sinusoidal modulation of the frequency sweep. The radar level gauge further comprises a second mixer configured to mix an intermediate frequency signal and an integer multiple of the frequency of the sinusoidal modulation to provide an adjusted intermediate frequency signal, and processing circuitry configured to determine the distance based on the adjusted intermediate frequency signal. 
     By providing a frequency sweep comprising a sinusoidal modulation, an improved sensitivity is achieved by the radar level gauge and method.

FIELD OF THE INVENTION

The present invention relates to a radar level gauge and method usingelectromagnetic waves to determine a distance to a surface of a productin a tank.

BACKGROUND OF THE INVENTION

Since radar level gauging was developed as a commercial product in the1970's and 1980's, frequency modulated continuous wave (FMCW) has beenthe dominating measuring principle for high accuracy applications. AnFMCW measurement comprises transmitting into the tank a signal which isswept over a frequency range in the order of a few GHz. For example, thesignal can be in the range 9-11 GHz, or 24-27 GHz. The transmittedsignal is reflected by the surface of the contents in the tank (or byany other impedance transition) and an echo signal, which has beendelayed a certain time, is returned to the gauge. The echo signal ismixed with the transmitted signal to generate a mixer signal, having afrequency equal to the frequency change of the transmitted signal thathas taken place during the time delay. Due to the linear sweep, thisdifference frequency, also referred to as an intermediate frequency(IF), is proportional to the distance to the reflecting surface. Themixer signal is often referred to as an IF signal.

More recently, the FMCW principle has been improved, and today typicallyinvolves transmitting not a continuous sweep but a signal with steppedfrequency with practically constant amplitude. When the transmitted andreceived signals are mixed, each frequency step will provide oneconstant piece of a piecewise constant IF signal, thus providing one“sample” of the IF signal. In order to determine the frequency of thepiecewise constant IF signal, a number of frequencies, N, greater than anumber stipulated by the sampling theorem will be required. The distanceto the reflecting surface is then determined using the frequency of theIF signal in a similar way as in a conventional FMCW system. Typicalvalues can be 200-300 IF periods at 30 m distance divided in 1000-1500steps. It is noted that also a continuous IF signal, resulting from acontinuous frequency sweep, may be sampled in order to allow digitalprocessing.

Although highly accurate, conventional FMCW systems (continuous as wellas stepped) are relatively power hungry, making them less suitable forapplications where power is limited. Examples of such applicationsinclude field devices powered by a two-wire interface, such as a 4-20 mAloop, and wireless devices powered by an internal power source (e.g. abattery or a solar cell).

One of the reasons for the relatively high power requirement ofconventional FMCW systems is the need for isolation between thetransmitted and received electromagnetic waves. Isolation may also beimproved thus increasing the sensitivity, for example by adding an extramodulation to the frequency sweep. However, such modulation typicallyrequires added components, i.e. an additional oscillator and an adder,and will therefore increase the complexity and power consumption of thedevice. Hence it would be desirable to improve the sensitivity todecrease the power consumption, without increasing the complexity of thedevice.

GENERAL DISCLOSURE OF THE INVENTION

With regards to the above-mentioned desired properties of a radar levelgauge, it is a general object of the present invention to enableimproved performance of a radar level gauge by improving thesensitivity, and to reduce the energy demand of a radar level gauge byenabling a lower emitted power.

The present invention is based upon the realization that by configuringelectronic components comprised in a radar level gauge to provideordinarily undesirable effects, those undesirable effects will actuallyprovide an added modulation, thereby enabling an increased sensitivityfor the radar level gauge.

According to a first aspect of the present invention, these and otherobjects are achieved by a radar level gauge for measurement of adistance to a surface of a product in a tank, the radar level gaugecomprising transceiver circuitry configured to transmit anelectromagnetic transmit signal and receive an electromagnetic returnsignal reflected from the surface, the transceiver circuitry comprises afrequency stabilizing feedback loop configured to generate theelectromagnetic transmit signal in the form of a frequency sweep, afirst mixer configured to mix the transmit signal and the return signalto provide a first intermediate frequency signal, a signal propagationdevice arranged to guide the transmit signal towards the surface, and toguide the return signal to the transceiver circuitry. The frequencystabilizing feedback loop is intentionally configured to generate anoscillation, forming a modulation of the frequency sweep. The radarlevel gauge further comprises a second mixer configured to mix the firstintermediate frequency signal and an integer multiple of the frequencyof the sinusoidal modulation to provide a second intermediate frequencysignal and processing circuitry configured to determine the distancebased on the second intermediate frequency signal.

According to a second aspect of the invention the objects are alsoachieved by a method for detecting a distance to a surface of a productkept in a tank. The method comprises generating a frequency sweep in afrequency stabilizing feedback loop intentionally configured to generatean oscillation, thereby forming a modulation of the frequency sweep,transmitting an electromagnetic transmit signal comprising saidfrequency sweep towards the surface and receiving an electromagneticreturn signal comprising the frequency sweep reflected at the surface,mixing the return signal with the transmit signal to provide a firstintermediate frequency signal, mixing the first intermediate frequencysignal and an integer multiple of the frequency of the modulation toprovide a second intermediate frequency signal and determining thedistance based on the second intermediate frequency signal.

The modulation formed by the oscillation in the feedback loop enables animproved sensitivity without adding components, and thereby complexityto the radar level gauge. The improved sensitivity is achieved throughproviding the intermediate frequency signal at a higher frequency thannormal. The intermediate frequency signal according to the presentinvention will comprise the frequency of the sinusoidal modulation andthe harmonics of that frequency (i.e. integer multiples of thesinusoidal modulation frequency) plus the ordinary intermediatefrequency. The higher frequency of the intermediate signal will suppressthe leakage from electronics, waveguides and also decrease the impact ofinterference caused by objects in close proximity to the signalpropagation device. The increased sensitivity may be as much as 10-20dB.

It should be understood that the second mixing which mixes the firstintermediate frequency signal with an integer multiple of the frequencyof the sinusoidal modulation, provides the second, adjusted intermediatefrequency signal which has a frequency comparable to an intermediatefrequency signal in an ordinary FMCW radar level gauge.

The present invention is based on a surprising new use of a frequencystabilizing feedback loop. Conventionally, such a feedback loop is usedto ensure a smooth and distinct change of frequency when steppingthrough a frequency sweep. Any oscillating behavior is accordinglysuppressed as far as possible. According to the present invention,however, the frequency stabilizing feedback loop is intentionallyconfigured such that it causes oscillation before adjusting to the newfrequency. By ensuring that the duration of this oscillation iscomparable to the step time or dwell time of each frequency step, thisoscillation can be used to provide an essentially sinusoidal modulationof the frequency. The frequency stabilizing feedback loop could thus bereferred to as a “frequency oscillation feedback loop”.

The frequency stabilizing feedback loop comprises control parameterswhich in an ordinary case will be used to set the frequency stepping tobecome as linear as possible and to contain no oscillation, i.e. have adistinct stepping when the frequency increases. The frequencystabilizing feedback loop bandwidth will affect the size (i.e. thefrequency range) of the modulation, which means that a larger bandwidthwill allow for larger sinusoidal modulations outside of the usuallydesired frequency stepping. Furthermore the bandwidth also defines thespeed of the control loop i.e. the time before the frequency stabilizingfeedback loop achieves a “lock” on a frequency, a smaller allowedbandwidth will increase the time before the frequency stabilizingfeedback loop achieves a lock, and vice versa a larger allowed bandwidthwill decrease the time before the frequency stabilizing feedback loopachieves a lock. The frequency at which the frequency stabilizingfeedback loop samples the input and output signal will determine thefrequency of the added sinusoidal modulation.

A relationship between the sampling frequency and the loop bandwidth canbe 10 or less, or even as low as 5 or less. This may be accomplished bysetting the sampling frequency less than 10 MHz, or even less than 5MHz. In one example, the sampling frequency of the feedback loop is only2 MHz. In another example, the sampling frequency of the feedback loopis only 250 kHz.

According to one embodiment of the invention, a relationship between astep time, or step dwell time, for each frequency step of the frequencysweep and the duration of the oscillation is selected such that themodulation is essentially continuous over the frequency sweep. Thereby,a high sensitivity at all portions of the frequency sweep is possible.

Further, by adjusting the step dwell time, and thus the steppingfrequency, a modulation frequency lower than the comparison frequencycan be generated by configuring the quotient of the comparison frequencyand the stepping frequency as a rational number. If the quotient of thecomparison frequency and the stepping frequency is an integer, themodulation frequency for the sinusoidal modulation will become equal tothe comparison frequency. By configuring the quotient of the comparisonfrequency and the stepping frequency as a fractional number, themodulation will repeat itself with a frequency equal to the fraction.For example, if the quotient of the comparison frequency and thestepping frequency is six point four, which is six and two fifths, themodulation pattern will repeat itself with a frequency that is one fifthof the comparison frequency. It should be noted that the previousnumbers are just examples, and in principle any arbitrary modulationfrequency can be generated within the scope of the present invention.

Further features of, and advantages with, the present invention willbecome apparent when studying the appended claims and the followingdescription. The skilled person realize that different features of thepresent invention may be combined to create embodiments other than thosedescribed in the following, without departing from the scope of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described inmore detail, with reference to the appended drawings showing anembodiment of the invention.

FIG. 1 is a schematic representation of a frequency sweep.

FIG. 2 is a schematic section view of a radar level gauge suitable forimplementing the present invention.

FIG. 3 is a schematic block diagram of a transceiver and processingcircuitry in the radar level gauge in FIG. 2, according to an embodimentof the present invention.

FIG. 4 is an alternative schematic block diagram of a transceiver andprocessing circuitry in the radar level gauge in FIG. 2, according to anembodiment of the invention.

FIG. 5 is a flow chart of the method according to an embodiment of thepresent invention.

FIG. 6 is a schematic block diagram of a microwave source comprising aphase frequency stabilizing loop in the transceiver in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present description, embodiments of the present invention aremainly described with reference to a radar level gauge having a freepropagating antenna for radiating and capturing an electromagneticsignal. It should be noted that this by no means limits the scope of theinvention, which is equally applicable to other signal propagatingdevices, including other free propagating antennas such as a rodantenna, a patch antenna, a fixed or movable parabolic antenna or aconical antenna, and wave guides, such as a still pipe, a transmissionline or a probe, such as a single-line probe (including a so-calledGoubau probe), a twin-line probe or a coaxial probe.

Further, in the following description, embodiments of the presentinvention are mainly described with reference to an FMCW radar levelgauge using a stepped frequency sweep. It is noted that the presentinvention is advantageous in any sampled FMCW, such as a FMCW using acontinuous frequency sweep, or even other types of radar systems usingfrequency sweeps.

The present invention may be easiest understood by viewing FIG. 1, whichschematically illustrates a plot 100 of a frequency sweep, wherein thefrequency is plotted against time. The continuous line 101 representsthe ideal stepped frequency sweep 101, wherein the frequency sweep islinear and each stepping of the frequency is shown as a distinct step toa higher frequency. The dashed line 102 represents an oscillation, whichmay be present due to inappropriate setting of the frequency feedbackstabilizing loop. The oscillation represented by the dashed line 102 mayprolong the time at each frequency step, since the oscillation willregister as an error before settling and then allowing the frequencyfeedback stabilizing loop to stepwise increase the frequency. Hence,time and energy is usually spent to minimize the oscillation representedby the dashed line 102 in order to enable the time at each frequencystep to be as short as possible. The dotted line 103 in FIG. 1represents an oscillation according to the present invention i.e. anadded sinusoidal modulation 103. The oscillation which provides thesinusoidal modulation 103 is achieved by configuring the frequencyfeedback stabilizing loop in a systematic manner. Note that theamplitude of the sinusoidal modulation 103 and its frequency are mereschematic representations in FIG. 1. As will be further elaborated onlater, a typical size of the frequency step is on the order of MHz. Theamplitude of the sinusoidal modulation 103 is typically of the order ofMHz, and the frequency of the sinusoidal modulation is typically 200-400kHz or less.

FIG. 2 schematically illustrates a radar level gauge 1 according to anembodiment of the present invention, comprising a measurementelectronics unit 2, and a signal propagating device, here a horn antenna3. The radar level gauge 1 is provided on a tank 5, which is partlyfilled with a product 6 to be gauged. The product 6 in the tank may be aliquid, a liquid gas, or even a solid, such as grain or plastic pellets.The FMCW measurement method provides a relatively high measurementsensitivity of the radar level gauge, enabling reliable measurementresults also when interfering objects are present in the tank. Byanalyzing transmitted signal S_(T) being radiated by the antenna 3towards the surface 7 of the product 6, and echo signal S_(R) travelingback from the surface 7, the measurement electronics unit 2 candetermine the distance between a reference position and the surface 7 ofthe product 6, whereby the filling level L can be deduced. It should benoted that, although a tank 5 containing a single product 6 is discussedherein, the distance to any material interface present in the tank 5 canbe measured in a similar manner. Furthermore the transmitted signalS_(T) being radiated by the antenna 3 towards the surface 7, oftencomprises a frequency spectrum of 9 to 11 GHz or a frequency spectrum of24 to 27 GHz.

As is schematically illustrated in FIG. 2, the electronics unit 2comprises a transceiver 10 for transmitting and receivingelectromagnetic signal, which here is connected to the antenna 3 via awave guide 9. It is noted that the antenna 3 optionally may be connecteddirectly to the transceiver circuitry, or be connected via a suitablesignal medium, such as a coaxial cable, The unit 2 further comprisesprocessing circuitry 11, which is connected to the transceiver 10 forcontrol of the transceiver and processing of signal received by thetransceiver to determine the filling level of the product 6 in the tank5. The processing circuitry 11 is also connected to a memory 12, storingany software required for the operation of the radar level gauge 1, andalso providing RAM used during operation.

The processing circuitry 11 is further connectable to externalcommunication lines 13 for analog and/or digital communication via aninterface 14. As an example, the communication between the communicationinterface 14 and an external control station (not shown) can be providedby a two-wire interface, which has a combined function of bothtransmitting the measurement result to the control station and receivingpower for operation of the gauge 1. Such a two-wire interface mayprovide a more or less constant power, and the measurement result can besuperimposed on the power voltage using a digital protocol, such asFieldbus Foundation or HART. Alternatively, the current in the lines isregulated in accordance with the prevailing measurement result. Anexample of such an interface is the 4-20 mA industrial loop, where thecurrent is regulated between 4 and 20 mA, depending on the measurementresult. Alternatively, the radar level gauge 1 may communicatewirelessly with the control station using e.g. a Wireless HART protocol,and use a local power supply (not shown) with batteries or other meansof scavenging energy for autonomous operation.

The interface 14 here includes power management circuitry, including apower store 15 for storing power during periods when the microwave unitis inactive, thereby enabling higher power consumption during periodswhen the microwave unit is active (i.e. during the sweep). With suchpower management, lower average power consumption may be achieved, whilestill allowing short periods of higher power consumption. The powerstore 15 may include a capacitance, and may be restricted by spacerequirements as well as intrinsic safety requirements (applying when thegauge 1 is arranged in the hazardous zone of a tank with explosive orflammable contents)

Although being shown as separate blocks in FIG. 2, several of thetransceiver 10, the processing circuitry 11 and the interface 14 may beprovided on the same circuit board, or even in the same circuit.

Referring now to FIG. 3, there is shown a more detailed block diagram ofthe transceiver 10 and processing circuitry 11 in FIG. 2 according to anembodiment of the present invention.

The transceiver 10 here includes a microwave source 21, in turncontrolled by timing circuitry 23 forming part of the processingcircuitry 11. The microwave source 21 is connected to the antenna 3 viaa power divider 24, and also to a first mixer 25. The power divider 24is arranged to connect a return signal from the antenna to the firstmixer 25, in order to allow the first mixer 25 to mix the transmittedsignal from the microwave source 21 with the return signal and providethe intermediate frequency signal. The first mixer 25 is connected to aband pass filter 26, which in turn is connected to a second mixer 35.The second mixer 35 is also connected to the microwave source 21 inorder to receive a signal representing integer multiples of thefrequency of the sinusoidal modulation and mix the intermediatefrequency signal with the signal from the microwave source 21 to providethe adjusted intermediate frequency signal. The second mixer 35 isfurther connected to an amplifier 27.

The processing circuitry 11 here includes, in addition to the timingcircuitry 23 mentioned above, a sampler 31 adapted to receive and samplethe signal from the amplifier 27. The sampler may comprise asample-and-hold circuit in combination with an A/D-converter, or berealized as a sigma-delta converter. The sampler 31 is controlled by thetiming circuitry to be synchronized with the measurement signal.Finally, the processing circuitry includes a level calculator block 34connected to the sampler 31.

While the elements of the transceiver 10 are typically implemented inhardware, and form part of an integrated unit normally referred to as amicrowave unit, at least some portions of the processing circuitry 11are typically embodied by software modules executed by an embeddedprocessor. The invention is not restricted to this particularrealization, and any implementation found suitable to realize the hereindescribed functionality may be contemplated.

Referring now to FIG. 4, there is shown a more detailed block diagram ofthe transceiver 10 and processing circuitry 11 according to anotherembodiment of the invention. The transceiver of FIG. 4 is similar to theembodiment shown in FIG. 3 until the signal path after the band passfilter 26. The band pass filter 26 of FIG. 4 is directly connected to anamplifier 27.

The processing circuitry 11 here includes, a first sampler 28 which isconfigured to receive a signal representing the frequency of thesinusoidal modulation from the microwave source 21, and a second sampler29 which is configured to receive the intermediate frequency signal fromthe amplifier 27. The first sampler 28 and the second sampler 29 arealso configured to receive a timing signal from the timing circuitry 23in order to synchronize the sampling with the measurement signal. Thesamplers, as described above, may comprise sample-and-hold circuits incombination with A/D-converters, or be realized as a sigma-deltaconverters. The sampled signals from the first 28 and the second sampler29 are fed to a second mixer 35, which is configured to provide theadjusted intermediate frequency signal. The second mixer 35 is connectedto a low pass filter 30, which in turn is connected to the levelcalculator block 34 configured to determine the distance.

It should be understood that several of the components in FIG. 4 whichare incorporated in the processing circuitry 11, compared to theirrespective placement in the embodiment shown in FIG. 3, are partly orwholly embodied as software modules or blocks. For example, the secondmixer 35 which is now receiving a sampled signal from the first sampler28 representing the frequency of the sinusoidal modulation, and a secondsignal from the second sampler 29 representing the intermediatefrequency signal may now mix these digitally. Thus providing thepossibility to further amplify or filter the signal through known signalprocessing algorithms. Further, with sufficient computing power from theprocessing circuitry it may be enough to sample the signal from theamplifier 27 in the second sampler 29 and provide the adjustedintermediate frequency signal through calculating the frequency of thesinusoidal modulation with signal processing algorithms such as fastFourier transforms or similar methods.

FIG. 5 illustrates a method for providing a measurement of a distance toa surface of a product according to an embodiment of the inventionthrough a flow chart. First, the steps S1-S6 of the method will bedescribed, and later steps S11 and S12 will be described in conjunctionwith FIG. 6.

First in step S1, the timing circuitry 23 controls the microwave source21 to output a measurement signal in the form of a stepped frequencysweep. The stepped frequency sweep is generated in the microwave source21 which comprises a frequency stabilizing feedback loop. Themeasurement signal can normally be stepped from a lower frequency to ahigher frequency in suitable steps. In an alternative embodiment themeasurement signal may instead be stepped from a higher to a lowerfrequency, or the frequency steps may even be taken in any order. As anexample, the frequency sweep may have a bandwidth in the order of a fewGHz (e.g. 0.2-6 GHz), and an average frequency in the order of 25 GHz or10 GHz. This number of steps N in the sweep may be in the range100-4000, typically 200-2000, and may be around 1000 for a desired rangeof 30 m. The size of each frequency step (Δf) will thus typically be inthe order of MHz. For a power limited application the duration of thesweep is limited, and is typically in the order of 0-100 ms. As anexample, the duration of the sweep may be around 30 ms, and with 1000frequency steps (N=1000), this results in a duration for each step inthe order to 30 μs, or an update rate of around 30 kHz. The frequencyfeedback stabilizing loop is configured to generate an oscillationforming a sinusoidal modulation of the stepped frequency sweep.

Secondly, in step S2, the measurement signal from the microwave source21 is emitted into the tank 5 as an electromagnetic transmit signalS_(T) by the antenna 3 towards the surface 7.

Then, in step S3, a return signal S_(R) traveling back from the surface7 after being reflected, is received by the antenna 3 and sent along thewaveguide 9 to the transceiver 10 and thus the power divider 24. In stepS4 the return signal S_(R) is sent via the power divider 24 to the firstmixer 25 and is mixed with the measurement signal to provide anintermediate frequency signal. Ordinarily the intermediate frequencysignal is a piecewise constant oscillating signal, with a frequencyproportional to the distance to the reflecting surface and the piecewiseconstant length is the same length as the measurement signals steplength. A typical frequency is in the order of kHz, e.g. less than 100kHz, and typically less than 15 kHz. An intermediate frequency signalaccording to the present invention however, will have a higher frequencywhich is proportional to the intermediate signal plus multiples of thesinusoidal modulation frequency, thereby requiring a second step ofmixing to demodulate the intermediate frequency signal. The integermultiples of the sinusoidal modulation frequency are created as theharmonics of the sinusoidal modulation frequency as a result from theBessel functions involved. It should be noted that the energytransmitted in the electromagnetic transmit signal S_(T) will be dividedamong these harmonics, the increase in sensitivity however will morethan make up for the transmitted energy being divided.

Hence, in step S5, the intermediate frequency signal from the firstmixer 25, having passed the band pass filter 26 which has been set allowintermediate frequency signals of a certain frequency corresponding toat least one of the integer multiples of the sinusoidal modulationfrequency, is mixed with an integer multiple of the frequency of thesinusoidal modulation by the second mixer 35 to provide an adjustedintermediate frequency. Therefore, the integer multiple whichcorresponds to the same frequency configured for the band pass filter 27is preferably used in order to demodulate and provide the adjustedintermediate frequency signal. This adjusted intermediate frequencysignal comprises a frequency proportional to the distance to thereflecting surface, and is in the order of kHz, e.g. less than 100 kHz,and usually less than 15 kHz.

As an alternative, the second mixer 35 may be embodied in the processingcircuitry as software, wherein the return signal S_(R) is mixed with themeasurement signal in the first mixer 25 and the intermediate frequencysignal is sampled in the processing circuitry 11 before being mixed inthe second mixer 35 to provide the adjusted intermediate frequencysignal in the processing circuitry. Moreover, if the second mixer 35 isimplemented as software the frequency of the sinusoidal modulation fromthe microwave source 21 to the second mixer 35 may not be needed. Byproviding the second mixer 35 as software, unused processing abilityavailable in the processing circuitry 11 may be used to provide thedemodulation of the intermediate signal into an adjusted intermediatesignal, the information inherently known in the processing circuitry 11of the timing signal from the timing circuitry 23 will be used in orderto demodulate the intermediate frequency signal without adding extracomponents to the system.

Then, in step S6, the amplified adjusted intermediate frequency signalis received by the processing circuitry 11, where it is sampled andA/D-converted by the sampler 31. The sampling frequency of theA/D-converter 30 is advantageously sufficiently close to the update rateof the measurement signal, in order to sample each step of themeasurement signal once and only once.

The sample vector resulting from the sampling is supplied to the levelcalculator block 34, which determines the frequency of the adjustedintermediate frequency signal based on the sample vector, and thendetermines the distance to the reflecting surface (and subsequently thefilling level of the product in the tank) based on the frequency of theadjusted intermediate frequency signal.

In FIG. 6 the microwave source 21 is illustrated in more detail. Themicrowave source 21 comprises a phase frequency detector 36, a loopfilter 37, a voltage controlled oscillator 38, a feedback frequencydivider 39 and a frequency divider 45. The microwave source 21 will, inuse, receive a signal 40 from the timing circuit which comprises atiming frequency. The timing frequency may be divided by the frequencydivider 45 before being detected by the phase frequency detector 36. Thephase frequency detector will detect and compare the frequency of thesignal 40 from the frequency divider 45 with the signal produced by thevoltage controlled oscillator 38 which is divided by the feedbackfrequency divider 39, and produce an error signal proportional to thedifference between the phases of the signal 40 from the frequencydivider 45 and the signal from the feedback frequency divider 39. Theerror signal is then filtered in the loop filter 37 and used to drivethe voltage controlled oscillator 38 which will create an output signal41 comprising an output frequency. The output signal 41 will be returnedto the phase frequency detector 36 through the frequency divider 39,thereby creating a negative feedback loop, hence if the output signals41 frequency would drift the error signal would increase and drive itback toward the correct value. By means of the feedback loop 36, 37, 38,and 39 the output signal 41 from the microwave source 21 is “locked” tothe input signal 40 from the timing circuit 23. It is noted that theoutput frequency usually is not equal to the input signal frequency. Thefrequency divider 39 will divide the output signal by an integer N or afractional value, and hence the output frequency may be stepped ininteger multiples (or any rational number by the fractional division) ofthe input signal 40 to provide a stepped frequency sweep as an outputsignal 41 from the microwave source 21.

Accordingly there are also parameters which may be set for the frequencystabilizing feedback loop shown in FIG. 6. One parameter is the feedbackloop bandwidth 42, which should be understood as setting the loop filter37 bandwidth. Another parameter for the frequency stabilizing feedbackloop is the comparison frequency, which is the frequency of the inputsignal 40 from the timing circuit 23. The comparison frequency willdetermine the sampling frequency of the phase frequency detector 36. InFIG. 6 a frequency divider 45 is shown which is able to divide thefrequency of the input signal 40 from the timing circuit 23, hence bycontrolling the integer or fractional division by the frequency dividersignal 46 the comparison frequency is controlled. Furthermore the phasemargin 43 of the loop filter 37 may be set to determine the stability ofthe feedback loop. Another parameter which is also set in the microwavesource 21 is the step dwell time 44 i.e. the amount of time before theinteger multiple of the frequency divider is increased and thus astepping of the output signal 41 frequency is achieved.

The feedback loop bandwidth 42 will control the size of the error signalproduced by the phase frequency detector 36, and as a result alsocontrol the speed of the control loop, due to the fact that a largerallowed error signal (i.e. a larger bandwidth) will result in less timebefore the output signals 41 frequency may settle to the intendedfrequency. Ordinarily a high sampling frequency is required to ensurethat the stepping of the stepped frequency sweep is distinct and thatthe oscillation is as short as possible. However, by setting thecomparison frequency lower by controlling the frequency divider 45 orthe input signal frequency from the timing circuit 23, such that thephase frequency detector 36 samples slower than desirable for a distinctand smooth stepping of frequencies, an oscillation will occur in thefrequency sweep. This oscillation may be understood as a phase frequencydiscrete sampling effect. Moreover if the previously mentionedcomparison frequency is as low as ten times the feedback loop bandwidth42, the larger time required for the output signal 41 frequency tosettle to an intended frequency together with the relatively lowsampling frequency will produce oscillation i.e. phase frequencydiscrete sampling effects.

The step dwell time 44 is the amount of time spent at each step in thestepped frequency sweep before the frequency divider 39 increases theinteger N to N+1 (or any fractional value), which means that thefrequency stabilizing feedback loop will try to acquire a “lock” on thenext step in the stepped frequency sweep. By setting the step dwelltime, a continuous oscillation may be provided in order to form asinusoidal modulation of the frequency sweep which is continuous.Moreover the phase margin 43 may be set to produce phase frequencydiscrete sampling effects or produce them in combination with thefrequency stabilizing feedback loop bandwidth and comparison frequency.

Now referring to FIG. 5 again, step S11 comprises setting at least oneof feedback loop bandwidth 42, comparison frequency, or the phase margin43 in order to cause an oscillation when settling to a new frequency.

As an example, the comparison frequency is set to 2 MHz, and the loopbandwidth 42 is set to 200 kHz. The phase margin is set to 35 to 70degrees, e.g. 50 degrees. Other values may also be adequate, but it istypically important that the comparison frequency (i.e. the samplingfrequency of the loop) is not too much greater than the bandwidth. In aconventional frequency stabilizing feedback loop the relationship can bein the order of 100. Here, the relationship should typically not be morethan a factor 10 or even less. According to some embodiments acomparison frequency in the range around 200 kHz is used.

Secondly step S12, comprises setting the step dwell time 44 to provide acontinuous oscillation and thus a continuous sinusoidal modulation.Further, as mentioned earlier, the step dwell time 44 together with thecomparison frequency may be used to control the frequency of thesinusoidal modulation in order to produce a sinusoidal modulation at, inprinciple, any frequency.

Thus, step S11 and S12 may be performed only once the radar level gaugeis installed, or as often as is needed such as once a month or evenprior to every frequency sweep if required.

The frequency stabilizing feedback loop of FIG. 6 may be wholly orpartly incorporated by a phase locked loop (PLL). Phase locked loops arecommercially available in many format such as analog or digital, withinteger or fractional frequency dividers 45. Furthermore, a commercialPLL may comprise a charge pump and/or a frequency divider 45. Thecomparison frequency divider will enable an easy control of thecomparison frequency by dividing the comparison frequency of an inputsignal before entering the feedback loop. Moreover, the frequency of theoscillation, i.e. the modulation frequency, is often available on anoutput pin of a commercial PLL which may advantageously be used toconnect the microwave source 21 and the second mixer 35.

Although the figures may show a specific order of method steps, theorder of the steps may differ from what is depicted. Also two or moresteps may be performed concurrently or with partial concurrence. Suchvariation will depend on the software and hardware chosen and ondesigner choice. All such variations are within the scope of thedisclosure. Likewise, software implementations could be accomplishedwith standard programming techniques with rule based logic and otherlogic to accomplish the various connection steps, processing steps,comparison steps and decision steps. Additionally, even though theinvention has been described with reference to specific exemplifyingembodiments thereof, many different alterations, modifications and thelike will become apparent for those skilled in the art. Variations tothe disclosed embodiments can be understood and effected by the skilledaddressee in practicing the claimed invention, from a study of thedrawings, the disclosure, and the appended claims. Furthermore, in theclaims, the word “comprising” does not exclude other elements or steps,and the indefinite article “a” or “an” does not exclude a plurality.

What is claimed is:
 1. A radar level gauge for determine a distance to asurface of a product in a tank, said radar level gauge comprising:transceiver circuitry configured to transmit an electromagnetic transmitsignal and receive an electromagnetic return signal reflected from saidsurface, said transceiver circuitry comprising a frequency stabilizingfeedback loop configured to generate said electromagnetic transmitsignal in the form of a frequency sweep, wherein said frequencystabilizing feedback loop is intentionally configured to generate anoscillation, causing a modulation of said frequency sweep, a signalpropagation device arranged to guide said transmit signal towards saidsurface, and to guide said return signal to said transceiver circuitry,a first mixer configured to mix said transmit signal and said returnsignal to provide a first intermediate frequency signal, a second mixerconfigured to mix said intermediate frequency signal with a signalhaving a frequency which is an integer multiple of the frequency of saidmodulation to provide a second intermediate frequency signal, andprocessing circuitry configured to determine said distance based on saidsecond intermediate frequency signal.
 2. The radar level gauge accordingto claim 1, wherein said frequency stabilizing feedback loop has a loopbandwidth and a sampling frequency, and wherein said oscillation iscaused by setting at least one of said loop bandwidth, and said samplingfrequency.
 3. The radar level gauge according to claim 2, wherein saidfrequency stabilizing feedback loop comprises a phase frequencydetector, said sampling frequency being a comparison frequency of saidphase frequency detector.
 4. The radar level gauge according to claim 2,wherein a relationship between said sampling frequency and said loopbandwidth is 10 or less.
 5. The radar level gauge according to claim 2,wherein a relationship between said sampling frequency and said loopbandwidth is 5 or less.
 6. The radar level gauge according to claim 2,wherein said sampling frequency is less than 10 MHz.
 7. The radar levelgauge according to claim 2, wherein said sampling frequency is less than5 MHz.
 8. The radar level gauge according to claim 1, wherein arelationship between a duration of said oscillation and a step time foreach frequency in said frequency sweep is such that said modulation isessentially continuous over said frequency sweep.
 9. The radar levelgauge according to claim 1, wherein said frequency stabilizing feedbackloop is realized by a phase lock loop (PLL).
 10. The radar level gaugeaccording to claim 1, wherein said modulation is essentially sinusoidal.11. The radar level gauge according to claim 1, wherein said frequencysweep extends from 9 GHz to 11 GHz.
 12. The radar level gauge accordingto claim 1, wherein said frequency sweep extends from 25 GHz to 27 GHz.13. A method for detecting a distance to a surface of a product kept ina tank, said method comprising; generating a frequency sweep in afrequency stabilizing feedback loop intentionally configured to generatean oscillation, thereby causing a modulation of said frequency sweep,transmitting an electromagnetic transmit signal comprising saidfrequency sweep towards said surface, receiving an electromagneticreturn signal comprising said frequency sweep reflected at said surface,mixing said return signal with said transmit signal to provide a firstintermediate frequency signal, mixing said first intermediate frequencysignal and an integer multiple of the frequency of said modulation toprovide a second intermediate frequency signal, and determining saiddistance, based on said second intermediate frequency signal.
 14. Themethod according to claim 13, wherein said frequency stabilizingfeedback loop has a loop bandwidth and a sampling frequency, and whereinsaid step of generating said frequency sweep further comprises settingat least one of said loop bandwidth and said sampling frequency.
 15. Themethod according to claim 13, wherein a relationship between saidsampling frequency and said loop bandwidth is 10 or less.
 16. The methodaccording to claim 13, wherein a relationship between said samplingfrequency and said loop bandwidth is 5 or less.
 17. The method accordingto claim 13, wherein said sampling frequency is less than 10 MHz. 18.The method according to claim 13, wherein said sampling frequency isless than 5 MHz.
 19. The method according to claim 13, wherein arelationship between a duration of said oscillation and a step time foreach frequency in said frequency sweep is such that said modulation isessentially continuous over said frequency sweep.
 20. The methodaccording to claim 13, wherein said modulation is essentiallysinusoidal.
 21. The method according to claim 13, wherein said frequencysweep extends from 9 GHz to 11 GHz.
 22. The method according to claim13, wherein said frequency sweep extends from 25 GHz to 27 GHz.